A wide variety of curable polysiloxane compositions are known. Many of these curable polysiloxane compositions are moisture-curable. The curable compositions can be used to prepare a wide range of surface coatings and surface treatments, ranging from release coatings to pressure sensitive adhesives. The moisture-curable polysiloxane compositions cure in the presence of moisture to form crosslinked materials. The moisture for curing is typically obtained from the atmosphere or from a substrate to which the composition has been applied, although it can also be added to the composition (for example, to enable curing in depth or in confinement).
Moisture-curable polysiloxane compositions usually comprise siloxane polymers having groups (for example, alkoxysilyl or acyloxysilyl moieties) that can react in the presence of moisture to form cured (that is, crosslinked) materials. Moisture-curable compositions comprising alkoxysilyl or acyloxysilyl functionality typically cure in two reactions. In the first reaction, the alkoxysilyl or acyloxysilyl groups hydrolyze in the presence of moisture and a catalyst to form silanol compounds having hydroxysilyl groups. In the second reaction, the hydroxysilyl groups condense with other hydroxysilyl, alkoxysilyl, or acyloxysilyl groups in the presence of a catalyst to form —Si—O—Si— linkages. The two reactions occur essentially simultaneously upon generation of the silanol compound. Commonly used catalysts for the two reactions include Bronsted and Lewis acids. A single material can catalyze both reactions. Generally, it is desirable that the hydrolysis and condensation reactions proceed quickly after the moisture-curable composition has been applied, for example, to a substrate. At the same time, however, the reactions must not occur prematurely, for example, during processing or storage.
A variety of approaches have been used for providing moisture-curable compositions that have acceptable cure rates without processing and storage difficulties. For example, two-part systems have been developed (one part comprising a functional siloxane polymer and the other part comprising a catalyst), with the two parts being mixed immediately prior to use. While this approach has been useful in small-scale applications, it has been less efficient for large-scale manufacturing, where delays caused by having to mix the two parts have been undesirable. Furthermore, coating operations must be completed expeditiously before the composition cures in the pot, and this has been difficult when working with large surface area substrates or a large volume of composition.
A related curing technology involves the condensation reaction between two silanol groups (—SiOH). The condensation reaction is, in effect, the second reaction of the moisture-curable system, in which the silanol groups are already present and need not be generated by hydrolysis of alkoxysilyl or acyloxysilyl groups. The condensation reaction also generates the same type of —Si—O—Si— linkage and generates a molecule of water as a byproduct. Many of the same types of catalysts and curing techniques applicable to moisture-curable siloxanes are applicable to this condensation as well.
A variety of catalyst systems for curable siloxane compositions have been developed that are storage stable and readily activatable. For example, photoactivatable nitrogen bases are described in U.S. Pat. No. 7,538,104 (Baudin et al.) and a series of patent applications have been filed using photoactivatable nitrogen bases as catalysts for silicone coating compositions: Ser. No. 61/360,068, “Curable-on-demand Polysiloxane Coating Composition”; Ser. No. 61/360,019, “Curable Polysiloxane Coating Composition”; Ser. No. 61/360,007, “Curable-on-demand Composition Comprising Dual Reactive Silane Functionality”; and Ser. No. 61/359,985, “Curable Composition Comprising Dual Reactive Silane Functionality”; all filed on Jun. 30, 2010.
Among the curable polysiloxane compositions known are pressure sensitive adhesive compositions. Pressure sensitive adhesive compositions are well known to those of ordinary skill in the art to possess properties including the following: (1) aggressive and permanent tack, (2) adherence with no more than finger pressure, (3) sufficient ability to hold onto an adherend, and (4) sufficient cohesive strength to be cleanly removable from the adherend. Materials that have been found to function well as pressure sensitive adhesives are polymers designed and formulated to exhibit the requisite viscoelastic properties resulting in a desired balance of tack, peel adhesion, and shear holding power. Obtaining the proper balance of properties is not a simple process.
Examples of curable polysiloxane compositions are presented in U.S. Pat. No. 4,370,358 (Hayes et al.) which describes ultraviolet light curable silicone pressure sensitive adhesives comprising an epoxy functional silicone polymer and a combination of standard MQ resins or epoxy functional MQ resins, which can be cured to an adhesive composition in the presence of certain bis(aryl)halonium salt catalysts; and US Patent Publication No. 2008/0300358 (Cook et al.) which describes a continuous process for producing silicone pressure sensitive adhesives which includes mixing (A) a hydroxyl-functional polydiorganosiloxane polymer, (B) a hydroxyl-functional polyorganosiloxane resin, and (C) a solvent, while heating the composition at a temperature above the vaporization point of the solvent and removing essentially all volatile species in an apparatus with a residence time sufficient for bodying ingredients (A) and (B).